It is advantageous in an outboard marine engine system to arrange the crank shaft of the engine to extend vertically because the output shaft of the engine typically extends vertically between the engine which is at the top of the engine system and the propeller which is at the bottom of the engine system and submerged underwater. As the number of cylinders of the engine is increased to the end of increasing the engine output, it becomes desirable to design the engine as a V-type engine and reduce the overall length of the crank shaft.
In a vertical engine having a vertical crank shaft, distributing air-fuel mixture to each cylinder is very important because of the gravity acting upon the air/fuel mixture could cause uneven distribution of the mixture between the upper cylinders and the lower cylinders. Therefore, it is advantageous to provide a separate carburetor to each of the cylinders.
When a V-type engine is applied as an outboard marine engine, the lateral width of the engine is desired to be reduced as much as possible. Therefore, if a separate carburetor is to be provided to each of the cylinders, accommodating the engine intake system in a limited space imposes a serious difficulty on engine design.
In order to reduce the overall width of the vertical engine, it is desirable to bring the exhaust manifold as close to the engine as possible. However if the exhaust manifold is brought close to the engine, the heat from the exhaust manifold could cause an undesirable heat distribution around the cylinders.
Another important factor to be considered in designing a V-type vertical engine is the layout of cooling water passages. In an outboard marine engine, cooling water is typically collected from the surrounding water by way of a water inlet provided in a lower part of the engine system and submerged underwater. The collected water is then circulated in the water jacket defined in the engine and cools various parts of the engine before it is expelled back to the surrounding water. Because warm water is smaller in density than cool water, it is advantageous to arrange cooling water passages in such a manner that the collected water is supplied into the water jacket from a lower part of the engine and the water warmed by the engine heat is expelled from an upper part of the engine.
Therefore, a cooling water passage is necessary for conducting the cooling water expelled from the upper part of the engine into the surrounding water. The necessity for this cooling water passage is an additional factor to be considered in designing a V-type vertical engine. If this passage is provided in the cylinder block, the structure of the cylinder block will be made excessively complicated and the accessibility of the cooling water passage system will be severely restricted.
However, arranging a conduit for this cooling water passage externally to the cylinder block or the cylinder head will cause the problem of interference with other accessory devices of the engine. Furthermore, if the conduit is passed along a tortuous path, the flow resistance of the conduit may become excessive and the engine power will be wasted for pumping cooling water through the cooling water passages.
Yet another factor to be considered in designing a V-type vertical engine is the arrangement of a thermostat valve in the cooling system of the engine. To the end of maintaining the temperature of the cooling water at an appropriate level, circulation of the cooling water must be controlled by a thermo-valve or a thermostat valve. If this thermostat valve is installed in a cooling water passage formed within the cylinder head, the structure of the cylinder head tends to be complicated. Also, the thermostat is desired to be as accessible as possible for possible servicing purpose.
The crank case of the engine should be kept in a slightly negative pressure condition to assure the return of lubricating oil from the cylinder heads and preventing the generation of back pressure acting upon the pistons. Furthermore, because part of the combustion gas produced in the combustion chambers of the engine leak into the crank case, there is a need to ventilate the interior of the crank case. Typically, in a four-stroke engine, a breather passage having a one-way valve therein is provided for establishing a communication between the crank case and the atmosphere. To the end of preventing lubricating oil from being expelled from the engine along with the crank case gas, the opening end of the breather passage is required to have a certain volume to reduce the flow speed of the breather gas.